Macrophages are the main phagocytic cells of animals and play a key role in the immune system. Macrophages bind and injest particles recognised as foreign by the immune system. Such particles include microorganisms.
The three microorganisms Salmonella typhimurium, Leishmania donovani and Mycobacterium bovis (BCG) are all intracellular pathogens of macrophages. Three separate groups of scientists had previously identified genes capable of controlling resistance and susceptibility to each of these microorganisms. The genes were designated respectively Ity, Lsh and Bcg. Subsequent work has led the scientists to conclude that Ity/Lsh/Bcg is a single gene and is expressed at the macrophage level (Ref 1).
Recently, Vidal et al (Ref 2) cloned a murine gene as the most likely candidate to be Lsh/Ity/Bcg. This gene has been termed the natural resistance-associated macrophage protein (Nramp) gene. A cDNA for Nramp was isolated from a pre B-cell cDNA library and sequenced. The amino acid sequence for the protein product was deduced from the nucleotide sequence and predicts a 53 kDa protein. On the basis of the deduced amino acid sequence, Vidal et al proposed as a function of the Nramp protein the transport of nitrate across the membrane of the intracellular vacuole of the macrophage containing the microorganisms. In the acid environment of this vacuole, the nitrate could be converted via nitrite to toxic nitric oxide thereby to enhance killing of the microorganisms.
The present applicants have isolated and sequenced a macrophageexpressed Nramp cDNA. Contrary to the teaching of Vidal et al the present applicants have found a different nucleotide sequence including a region encoding an additional amino acid sequence at the N-terminus. Surprisingly, the additional amino acid sequence includes structural features which may be responsible for protein-protein interactions essential in signal transduction pathways thereby suggesting that Nramp controls early amplification of transmembrane signalling in disease resistant macrophages by binding the SH3 domain of tyrosine kinases or other molecules.